Diversity of species assemblages of islands: Predictions and their test using tree species composition of shola fragments.

Based on all possible combinations of species thatcan theoretically exist on islands of different sizes,we estimated similarities among islands of equal sizein their species compositions. We found that similar-ity coefficients among islands of equal sizes increasesand the coefficients of variation for similarities de-creases with increase in the size of islands. Accord-ingly, it can be predicted that the speciescompositions of small fragments shall be more di-terse compared to large fragments. We tested thisprediction using the tree species composition of theshola fragments at BR Hills, Western Ghats. Simi-larities among sholas were measured as coefficientsof correlation between the frequencies of species inthem and also as proportions of species sharedamong sholas. Small sholas were less similar andshared fewer species among themselves, compared tomedium and large sholas. Also, small sholas showedhigher coefficient of variation for the correlationcoefficients and for proportion of species shared thanmedium or large sholas. In other words, species as-semblage diversity among sholas was found to de-crease with their size and large islands appeared toconverge towards a common assemblage of species.Based on our results, we argue that small fragmentsadd structural and functional heterogeneity to theecosystem and hence are as important as are thelarge fragments in conserving biodiversity.